1. Field of the Invention
The present invention relates to a bead filler rubber composition. More specifically, it relates to a bead filler rubber composition having a low Mooney viscosity (ML) and an excellent workability, and capable of providing a vulcanizate having an excellent flex cracking propagation resistance.
2. Description of the Prior Art
With the recent development of high-speed highways, etc., tires are required to have more durability to high speeds. It is well known that as the running speed of a tire becomes higher, a standing wave phenomenon tends to occur, which in its turn leads to the destruction of the tire. Enhancing the rigidity of the bead portion of the tire is considered to be an effective means of suppressing the standing wave phenomenon. Furthermore, increasing the modulus of elasticity of the bead portion is effective for reducing the rolling resistance of the tire. For these reasons, rubber having a high modulus of elasticity is desirable as a bead filler rubber.
On the other hand, if the standing wave phenomenon occurs, the bead portion undergoes remarkable deformation. Therefore, from the viewpoint of safety, the cracking propagation resistance of the bead filler rubber should be high enough to prevent easy destruction of the tire. That is, in order to improve the durability at high speed of the tire, the bead filler rubber should satisfy the following requirements; a high modulus of elasticity and an excellent cracking propagation resistance.
Various methods have been heretofore attempted to obtain a rubber having a high modulus of elasticity. A method comprising compounding a large amount of carbon black is undesirable because the knitting performance of rubber during working procedures is poor, an increase electric power load is experienced during mixing in a Banbury mixer, and the resultant compound exhibits a high ML which results in difficulties in molding a tire from the compound. A method comprising compounding a large amount of sulfur is disadvantageous in that blooming of the sulfur occurs and the physical properties, such as flex cracking propagation resistance, of the resultant rubber are remarkably deteriorated due to the increased crosslinking density thereof. Another method of providing rubber having a high modulus of elasticity is to add a thermoplastic resin. However, because the thermosetting resin has a poor compatibility with natural rubber or a diene rubber, when the thermosetting resin is compounded in a large amount, the production conditions are subjected to severe limitations, for example, an extended period of mixing is needed to attain a satisfactory dispersion state. In addition, because rolling stock containing a large amount of a thermosetting resin compounded thereinto is hard even in its unvulcanized state, a large load may be required in mixing and extruding the stock or the molding workability may become inferior. A method comprising merely compounding short fibers is disadvantageous in that, because of unsatisfactory bonding between the short fiber and the rubber, the resultant vulcanizate exhibits a low creep resistance and a reduced fatigue life.